The present invention is an advancement in the field of active phased array radar systems.
In advanced radar systems, new waveforms such as the monopulse (single pulse) doppler waveforms can achieve enhanced sensitivities. The monopulse doppler waveform is characterized by a high duty factor and low pulse repetition frequency (PRF). Doppler information can be obtained from the return of a single transmitted pulse. A related type of waveform is the pulse burst waveform. This waveform is similar to the monopulse doppler waveform, except that the pulse is divided into many short bursts. The advantage is that receive operations can occur during the pulse "off" times during the pulse period, instead of being blanked during the entire long transmit pulse as is the case for the monopulse waveforms.
Postulated scenarios employing such new waveforms include very long ranges and heavy electronic countermeasures (ECM). Monopulse doppler waveforms are especially effective in these scenarios. Although transmitter spectral purity, distortion, sidelobes, analog-to-digital (ADC) and processor quantization and processing requirements can be met, the main challenge in implementing arrays with this waveform and other advanced waveforms is precise transmit pulse shaping.
An exemplary monopulse doppler waveform is illustrated in FIGS. 1A and 1B. FIG. 1A illustrates the transmit enable status as a function of time, showing the high duty factor and low PRF of the waveform. FIG. 1B shows the power distribution as a function of time for an exemplary one of the transmitted pulses, showing the pulse shaping. It is known that such pulse shaping is desirable in phased array radars in look-down scenarios, since slow moving target detection would be degraded in detection of slow moving targets. Ground clutter from the DC doppler filter will contaminate the lower non-DC filters, unless such pulse shaping is employed. Shaping is also useful in some conventional waveform applications to reduce the long range clutter settling time.
Transmit pulse shaping must be very precise in order to achieve adequate mainlobe clutter rejection. Also, to minimize array prime power and cooling requirements, it is important that high efficiency (i.e., minimal energy use) be maintained at the tapered edges.
In an active phased array radar system, one method of generating shaped pulses is to continuously vary the transmit power of each active element over the pulse length. However, with this method, it is extremely difficult to control the element amplitudes with sufficient precision. Furthermore, there is likely to be incidental phase modulation which also degrades clutter rejection.
Another method of achieving pulse shaping is duty factor modulation. The idea is to form the desired long shaped pulse as a series of many subpulses. The center-to-center spacing of the subpulses is constant, but the subpulse widths are reduced away from the long pulse center. This technique allows more precise control of the effective amplitude taper. However, the efficiency is poor because the elements as likely to require full power even during the gaps between subpulses.
It is therefore an object of the invention to provide a means for precise and efficient transmit pulse shaping in an active phased array radar.